Failure recovery techniques are extremely important for communication networks such as a photonic network when considering how to ensure that a network will have high reliability. The term “failure recovery” refers to switching a path from a active path on which a failure has occurred to a standby path when a failure such as a link disconnection or a node failure occurs.
A representative example of a failure recovery technique is 1:1 protection. Since the 1:1 protection technique does not cause data to flow on a standby path during normal operation, the 1:1 protection technique is receiving attention as a method that can make the maximum use of bands when considering the current situation in which increasing amounts of data are being transmitted within networks.
1:1 protection includes cases in which part of a standby path is shared by a plurality of active paths. As shown in FIG. 1, standby path 1′ for active path 1 and standby path 2′ for active path 2 share the same channel on a link between C and F. In this case, when a failure occurs on active path 1 or on active path 2, because part of the standby path is shared by a plurality of active paths, it is necessary for a node(s) on the standby path to set a pass for the standby path.
In the example illustrated in FIG. 1, nodes C and F are required to set the path for the standby path. A switch within the nodes is set so that data flows on standby path 1′ when a failure occurs in active path 1, and flows on standby path 2′ when a failure occurs in active path 2.
When part of a standby path is shared by a plurality of active paths, it is thus necessary to set a switch within a node on the standby path in the process of failure recovery. The flow of a conventional failure recovery will now be described.
First, a node on the active path that has detected the failure of the active path, provides notification that a failure has occurred in the transmission terminal node on the active path. Upon receipt of the notification, the transmission terminal node transmits a switch setting request signal as far as the receiving terminal node on the active path along a standby path for the active path on which the failure has occurred. A node that has received the switch setting request signal switches its switch to the standby path. The receiving terminal node on the active path that has received the switch setting request signal last time, switches its switch so that data can be received using the standby path, and then transmits a switch switching completion signal as far as the transmission terminal node on the active path along the standby path. Since switching of switches has been completed at all nodes, excluding the transmission terminal node on the standby path, the transmission terminal node on the active path that has received the switch switching completion signal determines that preparations for sending data on the standby path have been completed. Then, the transmission terminal node on the active path switches its switch from the active path to the standby path and causes data to flow along the standby path. Thus, the failure recovery operation has been completed.
However, the conventional failure recovery is problematic in that the failure recovery can not be performed at a high speed because much time is required in switch settings and in confirming completion of the settings during the failure recovery operation.
In order to achieve a high-speed failure recovery, Patent Literature 1, for example, discloses a technique in which, a node on an active path, which detects the failure of the active path, transmits a failure notification signal through flooding. The term “flooding” herein refers to the fact that a certain node transmits a packet to its all adjacent nodes.
A node that has received the failure notification signal retains therein information regarding all active paths within the network and standby paths that correspond to the active paths. A node that has received the failure notification signal determines from the failure notification signal and the path information whether or not its own node is a node on a standby path corresponding to the active path, the failure of which is detected. If its own node is a node on the standby path, the node switches its switch to the standby path, and transmits the received failure notification signal through flooding.
On the other hand, if its own node is not a node on the standby path corresponding to the active path, the failure of which is detected, the node transmits the received failure notification signal through flooding without switching its switch. Thus, switch setting is performed based on the failure notification signal from the node that has detected the failure, and the transmission terminal node on the standby path finally receives the failure notification signal through flooding. The failure recovery operation is completed when the transmission terminal node on the standby path switches its switch to the standby path.
In the technique disclosed by Patent Literature 1, prior to the reception of the failure notification signal by the transmission terminal node on the active path, each node switches its switch in response to a failure notification signal that is transmitted through flooding from the node that has detected the failure, thus enabling a high-speed failure recovery.
Patent Literature 2 discloses a technique in which a node that has detected a failure refers to a path table to select an alternative route, and transmits a switching request to an upstream branch node over the alternative route, thus enabling a high-speed failure recovery.
Patent Literature 3 discloses a technique in which, when the same path as a path which was used by a selected standby path is routed, a node that has detected the failure does not transmit a failure notification directly to an upstream node(s), but provides an instruction to a downstream node(s) on the overlapped route to transmit the failure notification.